In a conventional treatment plant for treatment of wood or lumber with a treating agent such as chromated copper arsenate (CCA), a large pressure vessel is used which is filled with an appropriate dilute treatment liquid. Where two concentrations of dilute treatment liquid are selectively used, two separate tanks are provided with each tank containing one concentration of the liquid. For example, with CCA, typically a 1 percent solution or a 1.4 percent solution (0.25 or 0.40 pounds per cubic foot CCA) are contained in respective tanks. To make up additional solution for each tank, a concentrated tank of CCA (usually commercially available 50 percent CCA) is fluidly connected to each tank. In addition, a water tank is also fluidly connected to each tank. Appropriate amounts of CCA concentrate and water are then pumped to the respective tanks as needed.
The method of operation of a conventional wood treatment plant is as follows. Initially, the wood to be treated is placed in the pressure vessel and the pressure vessel is sealed. A vacuum is then drawn on the pressure vessel to draw the air out of the wood. Next, depending on which concentration of dilute treatment liquid is desired to treat the wood, one of the solution tanks is connected to a pump which pumps that solution into the pressure vessel. The pump is utilized to raise the pressure in the pressure vessel to a predetermined value. As the wood goes from a vacuum to a pressure environment, the spaces from which the air was initially drawn by the vacuum are filled with the treatment liquid. After sufficient pressurizing, the remaining treatment liquid not absorbed by the wood is conducted back to the appropriate work tank from which it came.
Conventional plants of this type are made relatively large, typically 50'.times.100', and for economical reasons must be constructed in situ. In addition, as explained above, two separate work tanks are needed and these usually have a fairly large volume sufficient to fill the pressure vessel loaded with wood for a number of cycles of operation. Therefore, because of the size of the conventional plant, these plants are located in the southern portion of the United States adjacent the primary source of treating wood, pine trees. From these plants, the treated wood is transported all over the country. Unfortunately, treated wood weighs almost 50 percent more than untreated wood. Therefore, the shipping costs of treated wood are significantly higher than that for untreated wood. In addition, the size of the conventional plant and the output necessary to make the plant economical are too large for a location in a northern area where untreated wood could economically be shipped and thereafter the heavier treated wood economically distributed to a local market.
Therefore, there has existed in the prior art a necessity for a smaller wood treating plant which could make use of economically shipped untreated wood to a designated local market and which could then economically distribute treated wood to the designated local market.